Liu Yun-Jun, Norberg Fredrik E B, Szilágyi Anna, De Paepe Rosine, Akerlund Hans-Erik, Rasmusson Allan G
Lund University, Department of Cell and Organism Biology, Sölvegatan 35B, SE-22362 Lund, Sweden.
Plant Cell Physiol. 2008 Feb;49(2):251-63. doi: 10.1093/pcp/pcn001. Epub 2008 Jan 8.
Plant mitochondria contain alternative external NAD(P)H dehydrogenases, which oxidize cytosolic NADH or NADPH and reduce ubiquinone without inherent linkage to proton pumping and ATP production. In potato, St-NDB1 is an external Ca2+-dependent NADPH dehydrogenase. The physiological function of this enzyme was investigated in homozygous Nicotiana sylvestris lines overexpressing St-ndb1 and co-suppressing St-ndb1 and an N. sylvestris ndb1. In leaf mitochondria isolated from the overexpressor lines, higher activity of alternative oxidase (AOX) was detected. However, the AOX induction was substantially weaker than in the complex I-deficient CMSII mutant, previously shown to contain elevated amounts of NAD(P)H dehydrogenases and AOX. An aox1b and an aox2 gene were up-regulated in CMSII, but only aox1b showed a response, albeit smaller, in the transgenic lines, indicating differences in AOX activation between the genotypes. As in CMSII, the increase of AOX in the overexpressing lines was not due to a general oxidative stress. The lines overexpressing St-ndb1 had consistently lowered leaf NADPH/NADP+ ratios in the light and variably decreased levels in darkness, but unchanged NADH/NAD+ ratios. CMSII instead had similar NADPH/NADP+ and lower NADH/NAD+ ratios than the wild type. These results demonstrate that St-NDB1 is able to modulate the cellular balance of NADPH and NADP+ at least in the day and that reduction of NADP(H) and NAD(H) is independently controlled. Similar growth rates, chloroplast malate dehydrogenase activation and xanthophyll ratios indicate that the change in reduction does not communicate to the chloroplast, and that the cell tolerates significant changes in NADP(H) reduction without deleterious effects.
植物线粒体含有交替性外 NAD(P)H 脱氢酶，这些酶可氧化胞质 NADH 或 NADPH，并还原泛醌，且与质子泵浦和 ATP 生成没有内在联系。在马铃薯中，St-NDB1 是一种依赖 Ca2+ 的外 NADPH 脱氢酶。通过在纯合的野生烟草品系中过表达 St-ndb1 以及共抑制 St-ndb1 和野生烟草 ndb1，对该酶的生理功能进行了研究。在从过表达品系中分离出的叶片线粒体中，检测到交替氧化酶（AOX）的活性较高。然而，AOX 的诱导作用比先前显示含有升高量的 NAD(P)H 脱氢酶和 AOX 的复合体 I缺陷型 CMSII 突变体中的要弱得多。aox1b 和 aox2 基因在 CMSII 中上调，但在转基因品系中只有 aox1b 有反应，尽管较小，这表明不同基因型之间 AOX 激活存在差异。与 CMSII 一样，过表达品系中 AOX 的增加并非由于普遍的氧化应激。过表达 St-ndb1 的品系在光照下叶片 NADPH/NADP+ 比值持续降低，在黑暗中水平变化不定，但 NADH/NAD+ 比值不变。相反，CMSII 的 NADPH/NADP+ 比值与野生型相似，而 NADH/NAD+ 比值较低。这些结果表明，St-NDB1 至少在白天能够调节细胞内 NADPH 和 NADP+ 的平衡，并且 NADP(H) 和 NAD(H) 的还原是独立控制的。相似的生长速率、叶绿体苹果酸脱氢酶激活和叶黄素比值表明，还原的变化不会传递到叶绿体，并且细胞能够耐受 NADP(H) 还原的显著变化而无有害影响。
